Abstract: In this paper, the ultrasonic testing principle and techniques of porosity of carbon fiber composite laminates was briefly reviewed from the perspective of testing practice. The technique of ultrasonic through transmission C-scan for porosity testing and evaluation of carbon fiber composite laminates was introduced in detail, including the determination of porosity standard test blocks, the preparation of porosity thickness attenuation curve, the determination of porosity evaluation threshold, the matters for attention in porosity evaluation, and the application effect of testing. Ultrasonic through transmission c-scan testing and evaluation of porosity of carbon fiber composite laminates were summarized and discussed. It was proved that the ultrasonic through transmission C-scan technique based on the porosity standard test blocks can realize the production line monitoring and evaluation for the 2% porosity of carbon fiber composite laminates, and can meet the requirements of engineering specifications.